1. Field of Invention
The invention relates to an automatic transmission that is installed in a vehicle and, more particularly, to arrangement of various components of a speed-change mechanism of a gear train of the transmission.
2. Description of Related Art
Demands for a multi-stage design have been placed on automatic transmissions not only to secure driveability but also to improve fuel consumption, which is indispensable to the conservation of energy. In order to satisfy such demands, the number of clutches and brakes and the number of speed-change elements per speed-change stage of a gear train need to be further reduced. Hence, U.S. Pat. No. 5,106,352 proposes a gear train that employs a planetary gear set composed of a minimum possible number of speed-change elements and establishes six forward speeds and one backward speed by means of three clutches and two brakes for operating the planetary gear set. In the gear train according to the patent, three clutches are used to suitably input an engine output rotation and a decelerated rotation thereof to a planetary gear set composed of four speed-change elements of the transmission as two different rotations, and two brakes are used to control stopping of two speed-change elements. In this manner, the gear train achieves six multi-staged speeds.
The six-speed automatic transmission of the aforementioned related art necessitates, as its feature, two clutches for transmitting an input that has been decelerated through a reduction planetary gear to two different speed-change elements of the planetary gear set respectively. Although this structure makes it possible to obtain favorable gear ratios of six speeds, it is necessary to secure torque capacities of the two clutches and power transmitting members thereof, namely, a high-torque transmission system for the purpose of transmitting a torque that has been amplified through deceleration.
Further, because of a characteristic of the gear train, the two clutches for inputting decelerated rotation rotate at a high speed at a certain speed-change stage. Therefore, the rigidity of the two clutches must be further increased not only to transmit a high torque but also to stand high-speed rotation. In this respect, according to the structure of the related art, since the two clutches for inputting of decelerated rotation are arranged on one side of the planetary gear set, a member coupling one of the clutches to the planetary gear set passes through an outer space of the other clutch. If such a member is arranged on the outer space of the other clutch, the centrifugal force resulting from high-speed rotation further increases. The rigidity of the member coupling one of the clutches to the planetary gear set needs to be further enhanced, which causes an increase in size and weight.
If the high-torque transmitting member that rotates together with the planetary gear set becomes long and heavy, the inertia torque increases correspondingly. Therefore, the speed-change controllability deteriorates and speed-change shock may be caused.
The invention has been made in consideration of such circumstances. It is an object of the invention to provide a vehicular automatic transmission wherein the enlargement of the mechanism resulting from a multi-staged design is averted and the speed-change controllability is enhanced through reduction in weight and size of a high-torque transmission system by an ingenious arrangement of two clutches for inputting of decelerated rotation with respect to a planetary gear set.